In the aerospace industry there is continual search for stronger and lighter weight materials for use in the manufacture of diverse products. For several years industry has used composite materials comprising epoxy reinforced with various types of fibers such as glass, nylon, Dacron, boron and graphite.
In 1972 DuPont introduced a new family of aromatic polyamide fibers under the name Kevlar. These fibers were assigned the generic name of "Aramid" by the F.D.A. The new Aramid are available in several different types. Kevlar 49, an extremely tough fiber has a high tensile strength of 430,000 p.s.i., approximately 20 times that of nylon and 10 times that of Dacron. The fiber elongation to break is quite low and the density, while slightly higher than nylon or Dacron, is 43% lower than glass. Kevlar 49 has a tensile strength approximately equal to steel at only one-fifth the weight. Weight savings of up to 30% have been realized for composites made with Kevlar 49 over composites reinforced with glass fibers. Considering all these beneficial properties, substantial new applications are possible for composite parts made of epoxy reinforced with Kevlar 49 fibers.
In order to use composite parts reinforced with Kevlar 49 fibers and make them into integral structures, holes must be drilled in the parts to receive screws, bolts and other fasteners. Due to the extreme toughness of the Kevlar 49 fibers, unusual problems are encountered in drilling the composite parts. Various types of drills have been tried on the Kevlar 49 fiber reinforced composites. While drills have been able to penetrate the composites, they have caused the composites to delaminate, and have not produced neat clean-cut holes.
Conventional drills having a center leading screw draw or pull the cutting edges of the drill into the composite part. This center leading screw bends and pushes the Kevlar fibers away from the drill point and interior cutting surfaces, preventing the fibers from being neatly sheared off. The resulting holes have fibers protruding from the sides and the composite part may be delaminated on the entrance or exit side of the part adjacent to the hole.
Some prior art drills have eliminated the center leading screw and rely upon sharp spur points on the edges of the drill to scribe a hole in the workpiece--see, for example, U.S. Pat. No. 283,040. In tests on Kelvar components the leading circumferential spurs on this drill scribed a hole and partially cut the reinforcing fibers, however, the drill has not produced satisfactory clean-cut holes because the interior cutting surfaces at the central portion of the drill cut from the inside of the hole adjacent the axis of the drill outward and tended to push the reinforcing Kevlar fibers away from the cutting surfaces near the axis of the drill producing a fuzzy and uneven hole with delamination of the part.
The toughness of the Kevlar 49 fibers requires that an inward shearing action take place when the drill penetrates the composite rather than an outward shearing action used on other materials. The fibers must be cut by first cutting the hole periphery and then the fibers must be sheared from the periphery of the hole inward toward the axis of the hole to remove the center plug.